1. Field of the Invention
The invention relates to a polyaxial bone anchoring device for anchoring a stabilization rod relative to a bone or a vertrebra. The bone anchoring device includes an anchoring element, a receiving part for receiving a head of the anchoring element and for receiving a stabilization rod to be connected to the anchoring element. The anchoring element is pivotably connected to the receiving part and can be locked at an angle by exerting pressure onto the head via a pressure element that is arranged in the receiving part. The pressure element and the receiving part are configured to cooperate in such a way that the pressure element can frictionally clamp the head to maintain a desired angular position before locking of the polyaxial bone anchoring device.
2. Description of Related Art
U.S. Pat. No. 5,716,356 describes a polyaxial bone screw including a screw element and a receiving part which is pivotably connected to the screw element and a pressure element to exert pressure onto the head of the screw element to lock the angle between the screw element and the receiving part. The receiving part has a U-shaped channel for receiving a stabilization rod. The pressure element comprises a cylindrical recess, which is to be aligned with the U-shaped channel to receive the rod therein. In order to hold the pressure element in a position aligned with the U-shaped channel, the position of the pressure element is fixed by crimping through bores provided in the receiving part.
However, when the head of the bone anchoring element remains freely pivotable with respect to the receiving part before locking the head in a final angular position, alignment of the receiving part and the insertion of the rod may be difficult in more complex clinical applications, for example, when a multitude of bone anchors have to be connected to the rod.
U.S. Pat. No. 7,604,656 (Shluzas) describes a fastener engageable with a bone portion to connect a longitudinal member to the bone portion. The housing that receives the fastener also receives a spacer, which is engageable with the fastener and the longitudinal member. In one embodiment, the spacer is urged by a pin member into frictional engagement with the fastener and with the housing.